U.S. patent number 6,246,874 [Application Number 09/213,027] was granted by the patent office on 2001-06-12 for method and apparatus for predicting spot beam and satellite handover in a mobile satellite communication network.
This patent grant is currently assigned to Hughes Electronics Corporation. Invention is credited to Daniel A. Voce.
United States Patent |
6,246,874 |
Voce |
June 12, 2001 |
Method and apparatus for predicting spot beam and satellite
handover in a mobile satellite communication network
Abstract
A method and apparatus for predicting when to perform spot beam
and satellite handover in a mobile satellite communication network
that uses the position of the mobile subscriber. The method and
apparatus perform all calculations with respect to a
satellite-based coordinate system, thereby eliminating the need to
model the shape of the geometrically complex spot beams on the
surface of the earth. The position of a mobile subscriber unit that
initiates a call is tracked relative to a set of spot beam
boundaries that are located equidistant between a first spot beam
within which the subscriber unit is located at the time of call
initiation and a set of adjacent spot beams. An interval during
which the subscriber unit will cross over one of the boundaries is
estimated. The spot beam into which the subscriber is traveling is
identified, and the interval is adjusted until a desired level of
accuracy has been achieved at which time the call is transferred
from the first spot beam to the adjacent spot beam at the estimated
time. For satellite handover, an angle of elevation between the
subscriber unit and the satellite is calculated and compared to a
threshold angle. When the angle of elevation drops below the
threshold angle due to the movement of the satellite, the call is
transferred from a first satellite to a neighboring second
satellite.
Inventors: |
Voce; Daniel A. (Germantown,
MD) |
Assignee: |
Hughes Electronics Corporation
(El Segundo, CA)
|
Family
ID: |
26769354 |
Appl.
No.: |
09/213,027 |
Filed: |
December 16, 1998 |
Current U.S.
Class: |
455/428;
455/13.1; 455/439; 455/440 |
Current CPC
Class: |
H04B
7/18571 (20130101); H04B 7/18541 (20130101); H04B
7/1855 (20130101); H04W 36/00 (20130101); H04W
84/06 (20130101); H04W 64/006 (20130101) |
Current International
Class: |
H04B
7/185 (20060101); H04Q 007/20 (); H04B
007/185 () |
Field of
Search: |
;455/427,121,436,440,428,439,13.1,13.2 ;370/316 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Trost; William G.
Assistant Examiner: Perez-Gutierrez; Rafael
Attorney, Agent or Firm: Whelan; John T. Sales; Michael
W.
Parent Case Text
This application claims the benefit under 35 U.S.C. .sctn. 119(e)
of the filing date of Provisional Application No. 60/083,482, filed
Apr. 29, 1998.
Claims
What is claimed is:
1. A satellite communication system adapted to transfer a call from
a first spot beam to a second spot beam wherein the call is being
transmitted between a satellite and a subscriber unit located in
the first spot beam, the system comprising:
a first processor, the first processor being adapted to:
track a movement of the subscriber unit relative to a satellite
based coordinate system;
identify the second spot beam from among a plurality of spot
beams;
estimate a time at which to transfer the call that is dependent on
the position of the subscriber unit relative to the satellite based
coordinate system; and
generate a signal to cause the satellite to transfer the call at
the time;
a second processor being adapted to respond to the signal generated
by the first processor;
a first antenna being associated with the satellite fore generating
the first spot beam; and
a second antenna being associated with the satellite for generating
the second spot beam;
wherein the first antenna and the second antenna are controlled by
the second processor; and
wherein the system is further adapted to transfer the call from a
first satellite to a second satellite wherein the first processor
is further adapted to
(a) determine the position of the subscriber unit at the estimated
time;
(b) determine the position of the satellite at the estimated
time;
(c) calculate an angle of elevation using the position of the
subscriber unit and the position of the satellite;
(d) compare the angle of elevation to a threshold angle to
determine whether the subscriber unit is moving out of the first
footprint into a second footprint that is generated by the second
satellite;
(e) substitute a new time for the estimated time, if the angle of
elevation is not less than the threshold angle and thereafter
repeat steps (a)-(d); and
(f) cause the first satellite to transfer the call to the second
satellite when the angle of elevation is less than the threshold
angle.
2. A method for determining when to transfer a call associated with
a first spot beam to a second spot beam in a satellite
communication system, wherein the call is being transmitted
between, inter alia, a non-geostationary satellite and a subscriber
unit located in the first spot beam, the steps comprising:
tracking a movement of the subscriber unit relative to a satellite
based coordinate system,
estimating a transfer time at which to transfer the call based on
the movement of the subscriber unit relative to the satellite based
coordinate system,
causing the satellite to transfer the call at the estimated
transfer time, and
determining when to transfer the call from a first satellite to a
second satellite,
wherein the call is being transmitted from the first satellite to a
subscriber unit located in a first footprint, the first footprint
being generated by the first satellite, and
wherein the steps for determining when to transfer the call from
the first satellite to the second satellite comprise:
(a) determining the position of the subscriber unit relative to the
satellite based coordinate system at the estimated time;
(b) calculating an angle of elevation using the position of the
subscriber unit and a position of the satellite;
(c) comparing the angle of elevation to t threshold angle to
determine whether the subscriber unit is moving out of the first
footprint into a second footprint, the second foot print being
generated by the second satellite;
(d) substituting a new time for the estimated time, if the angle of
elevation is not less than the threshold angle and thereafter
repeating steps (a)-(c); and
(e) causing the first satellite to transfer the call to the second
satellite when the angle of elevation is less than the threshold
angle.
3. A satellite communication system adapted to transfer a call from
a first spot beam to a second spot beam wherein the call is being
transmitted between a satellite and a subscriber unit located in
the first spot beam, the system comprising:
a first processor, the first processor being adapted to:
track a movement of the subscriber unit relative to a satellite
based coordinate system;
identify the second spot beam from among a plurality of spot
beams;
estimate a time at which to transfer the call that is dependent on
the position of the subscriber unit relative to the satellite based
coordinate system; and
generate a signal to cause the satellite to transfer the call at
the time;
a second processor being adapted to respond to the signal generated
by the first processor;
a first antenna being associated with the satellite fore generating
the first spot beam;
a second antenna being associated with the satellite for generating
the second spot beam;
a transceiver for receiving signals transmitted by the first
processor and for routing the signals to the second processor;
and
a frequency translator for converting the signals received by the
transceiver to a format suitable for controlling the first antenna
and the second antenna;
wherein the first antenna and the second antenna are controlled by
the second processor.
4. A method for determining when to transfer a call associated with
a first spot beam to a second spot beam in a satellite
communication system, wherein the call is being transmitted
between, inter alia, a non-geostationary satellite and a subscriber
unit located in the first spot beam the steps comprising:
tracking a movement of the subscriber unit relative to a satellite
based coordinate system,
estimating a transfer time at which to transfer the call based on
the movement of the subscriber unit relative to the satellite based
coordinate system, and
causing the satellite to transfer the call at the estimated
transfer time,
wherein a boundary that is located between the first spot beam and
the second spot beam is defined relative to the satellite based
coordinate system,
the step of estimating a transfer time comprises estimating an
interval of time during which the subscriber unit will cross over
the boundary, and
the step of estimating an interval of time further comprises
repeatedly adjusting the interval of time until the interval of
time conforms to a desired level of accuracy.
5. A method as defined in claim 4 wherein the interval of time ends
at an endpoint and wherein the step of adjusting the interval
comprises:
calculating a position of the subscriber unit at the endpoint of
the interval;
comparing the position of the subscriber unit at the endpoint to
the boundary to determine whether the subscriber unit has crossed
over the boundary during the interval;
substituting a new value for the endpoint of the interval if the
subscriber unit has not passed over the boundary during the
interval; and
narrowing the interval if the subscriber unit has passed over the
boundary during the interval;
repeating the steps of calculating, comparing, substituting and
narrowing until the interval of time conforms to a desired level of
accuracy.
6. A method as defined in claim 5 wherein the boundary is located
equidistant from a center of the first spot beam and from a center
of the second spot beam, and wherein the step of comparing
comprises comparing the distance between the position of the
subscriber unit at the endpoint and the center of the first spot
beam to the distance between the position of the subscriber unit at
the endpoint and the center of the second spot beam.
7. A method for determining when to transfer a call associated with
a first satellite to a second satellite in a satellite
communication system wherein the call is being transmitted from the
first satellite to a subscriber unit located in a first footprint,
the first footprint being generated by the first satellite, the
method comprising of the steps of:
a) estimating a time at which to transfer the call;
b) determining the position of the subscriber unit relative to a
satellite based coordinate system at the estimated time;
c) calculating an angle of elevation using the position of the
subscriber unit and a position of the satellite;
d) comparing the angle of elevation to a threshold angle to
determine whether the subscriber unit is moving out of the first
footprint into a second footprint, the second footprint being
generated by the second satellite;
e) substituting a new time for the estimated time, if the angle of
elevation is not less than the threshold angle and thereafter
repeating steps b-d;
f) causing the first satellite to transfer the call to the second
satellite when the angle of elevation is less than the threshold
angle.
8. A method as defined in claim 7 wherein the step of estimating
the time at which to transfer the call and the step of determining
the position of the subscriber unit at the estimated time comprise
using a velocity of the satellite and an orbital path of the
satellite.
Description
(A) FIELD OF THE INVENTION
This invention relates generally to mobile satellite communication
networks and, more particularly, to a method for predicting spot
beam and satellite handover that utilizes the position of the
mobile caller relative to the position of the satellite to estimate
the time at which handover should occur.
(b) DESCRIPTION OF THE RELATED ART
Terrestrial cellular communication systems affect cellular
communication in a geographic region via a plurality of stationary
transmission towers each of which provides service to an individual
service area commonly referred to as a cell. Each cell typically
has a diameter in the range of several kilometers. To ensure
continuous service to mobile subscribers traveling throughout the
geographic region, the towers are positioned in a manner such that
the cells are adjacent to and overlap with six other cells provided
that the cells are not located on the boundary or edge of the
service coverage area. Cells located on the edge of the service
coverage area, i.e., an edge cell, may overlap with fewer than six
other cells.
In land-based cellular communication systems, the decision to
transfer a call from one service area to an adjacent service area
(a procedure known as handover or handoff) primarily involves
consideration of the signal quality of the on-going call.
Typically, a stationary tower servicing a given service area (i.e.,
a cell) includes equipment to monitor the signal quality of the
on-going call. When the measured signal quality decreases beneath a
predetermined threshold, the tower transfers the call to the tower
that services the adjacent cell into which the mobile subscriber is
moving.
Although this method is acceptable in terrestrial cellular systems,
the use of signal quality measurement as a means for determining
when to effect handover can be subject to large errors in the
satellite environment. These errors are typically caused by
instability or other errors present in the signal that is
transmitted by the earth station for subsequent relay to the mobile
unit via the satellite. In particular, if an earth station based
modem transmits control signals to a satellite beam at a power
level that is different from the nominal level because of
instability or other errors, the mobile unit that receives the
transmitted signals may erroneously register its position as being
inside the beam when in fact it may be outside of the beam or vice,
versa.
A handover method has been proposed that involves tracking the
mobile subscriber's position relative to the nominal cell
boundaries to determine when handover should occur. In simplified
terms, the method involves modeling the cells and cell boundaries
on the earth's surface, then tracking the subscriber's movement
relative to the earth based model. Thus, handover is performed when
a subscriber crosses a cell boundary thereby moving out of a first
cell and into an adjacent, second cell. The method becomes more
complicated, however, because, to ensure continuous service
coverage, adjacent cells slightly overlap each other. Due to the
overlapping regions of adjacent cells, a mobile user may occupy a
position that lies in two or three cells simultaneously. To
determine when cell handoff should occur for a subscriber located
in the overlapping region, it has been proposed that the cells be
modeled using inscribed hexagons wherein the sides of the hexagons
bisect the overlapping regions of the cells. The hexagons do not
overlap but instead are placed in an abutting manner so that, when
viewed together, they form a honeycomb-like grid. Thus, the hexagon
boundaries are lines of demarcation used, in relation to the
subscriber, to determine when handoff is to occur. For example, a
call made by a subscriber located within the boundaries of a
hexagon is serviced by the cell corresponding to that hexagon. When
the mobile subscriber travels into an adjacent hexagon, then the
call is handed over to the cell corresponding to this adjacent
hexagon.
This proposed method of tracking a mobile subscriber's position
relative to an earth based modeling system has several drawbacks
when used in the non geo-stationary satellite environment. For
example, in a non geo-stationary satellite communication system, a
direct radiating antenna projects a plurality of circularly shaped
spot beams onto the earth. Each spot beam represents a single
service area or cell, such that a call made by a subscriber located
within the boundaries of the cell are serviced by the corresponding
spot beam. However, the spot beams, when mapped onto the surface of
the earth, do not have a circular shape but instead have an
elliptical shape due, in part, to the angle at which the satellite
projects the beams onto the earth and also due to the spherical
shape of the earth. Because of the elliptical shape of the spot
beams, the inscribed hexagons used to model the spot beams and
demarcate the cell boundaries are irregularly shaped, thereby
making it very difficult to track the position of the subscriber
relative to the irregular boundary. In addition, unlike the
stationary hexagons used in a land-based system, the hexagons in
the non geo-stationary satellite system are typically hundreds of
kilometers in diameter and move over the surface of the earth in
conjunction with the spot beam antenna on the satellite. To further
complicate matters, various points on the boundaries of the
hexagons move at different speeds. Thus, complex and time consuming
geometrical procedures are required to model the rapid movement of
the large, irregularly shaped hexagons. Moreover, the spherical
shape of the earth causes the hexagons to be particularly distorted
at the North and South poles so that the complex modeling
procedures routinely lack precision in these areas.
Thus, there is a need in the art of satellite communications for a
simplified method for predicting when handover should occur that is
not subject to the polar sensitivity seen in the existing and
proposed methods.
SUMMARY OF THE INVENTION
In one aspect, the present invention is directed to a method for
performing handover in a satellite communication system. In
particular, the method is used to determine when to transfer a call
associated with a first spot beam to a second spot beam in a
satellite communication system wherein the call is transmitted
between a non geo-stationary satellite and a subscriber unit
located in the first spot beam. The movement of the subscriber unit
is tracked relative to a satellite based coordinate system and the
movement of the subscriber is used to estimate a time at which the
satellite will subsequently transfer the call.
In another aspect of the invention, a boundary is located between
the first and the second spot beams and an interval of time during
which the subscriber will cross over the boundary is estimated. The
interval of time is repeatedly adjusted until the interval conforms
to a desired level of accuracy.
To adjust the interval, the position of the subscriber within the
satellite coordination system is calculated at the end of the
interval and then compared to the location of the boundary to
determine whether the subscriber has crossed over the boundary
during the interval. If the subscriber has not passed over the
boundary, then a new value is selected as the end of the interval.
If instead, the subscriber has passed over the boundary, then the
interval may be narrowed to obtain a more accurate estimate of the
interval.
In yet another aspect of the present invention, the position of the
subscriber and the position of the satellite are determined
relative to an earth based coordinate system. The position of the
subscriber is then converted using a transformational matrix such
that the converted position is expressed relative to the satellite
based coordinate system.
In yet another aspect of the present invention, a method is
provided for determining when to transfer a call associated with a
first satellite to a second satellite in a satellite communication
system wherein the call is being transmitted from the first
satellite to a subscriber unit located in a first footprint that is
generated by the first satellite. A time at which to transfer the
call is estimated, and the position of the subscriber and the
position of the satellite at the estimated time are determined. An
angle of elevation is calculated using the position of the
subscriber and the position of the satellite. The angle is compared
to a threshold angle to determine whether the subscriber is moving
out of the first footprint into a second footprint that is
generated by the second satellite. If the angle of elevation is not
less than the threshold angle, a new time is estimated, and the
foregoing steps are repeated. If instead the angle of elevation is
less than the threshold angle, the call is transferred from the
first satellite to the second satellite.
The invention itself, together with further objects and attendant
advantages, will best be understood by reference to the following
detailed description, taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a satellite mobile communication
system used to implement the method of present invention;
FIG. 2 is a block diagram illustrating further details of the earth
station shown in FIG. 1;
FIG. 3 is a block diagram illustrating further details of the
satellite shown in FIG. 1;
FIG. 4 illustrates a satellite projecting two spot beams onto the
earth;
FIG. 5 illustrates two neighboring satellites projecting satellite
footprints onto the earth;
FIG. 6 illustrates the angle of elevation calculated in accordance
with the present invention;
FIG. 7 is a flow chart showing the steps of the method of the
present invention;
FIG. 8 illustrates the position of the subscriber and the spot
beams projected onto the x-y plane of the satellite-based
coordinate system.
DETAILED DESCRIPTION
Referring to FIG. 1, which illustrates a satellite communication
system 9 for use with the present invention, a low to medium earth
orbit non geo-stationary satellite 10 relays communication signals
from a mobile subscriber unit 11 (e.g., a radio telephone in an
automobile) to an earth based feeder antenna 12 and vice versa. The
feeder antenna 12, in turn, communicates with an earth station 14
that is coupled to a terrestrial network that includes the public
switched telephone network ("PSTN") 16. As shown in FIG. 2, the
earth station 14 includes a modem 18 that communicates with the
feeder antenna 12 and also communicates with a processor 20 and
associated memory 21 via a modem modulator/demodulator 22.
Although, for simplicity, FIG. 1 only shows a single satellite 10,
earth station 14 and feeder antenna 12, it is understood that the
satellite communication system 9 additionally includes any number
of satellites (e.g., ten) positioned in preselected orbits to
provide continuous overlapping coverage of the earth's surface and
a global network of earth-station 14 and feeder-antenna 12 pairs
that are interconnected by high speed terrestrial links. The
network of earth stations 14 is also linked to a central satellite
control center 15 (shown in FIG. 1) that tracks the position of the
satellites 10 and provides satellite orbital information such as
satellite position and velocity to the earth stations 14.
FIG. 3 is a diagram illustrating various aspects of the satellite
10 shown in FIG. 1. As shown in FIG. 3, each satellite 10 includes
a direct radiating array antenna ("DRA") 24 that supports
communication between the mobile subscriber unit 11 and the
satellite 10. A feeder link antenna 26 supports communication
between the satellite 10 and the earth station 14. The feeder link
antenna 26 and DRA 24 are linked to a satellite processor 28 having
associated memory 30. In operation, signals originating from the
earth station 14 are transmitted by the earth based antenna 12 to
the feeder link antenna 26. The signals are then relayed to the
mobile subscriber unit 11 via the DRA 24. In addition, signals
transmitted by the mobile subscriber unit 11 are received at the
DRA 24 and then relayed to the earth station 14 via the feeder link
antenna 26. Because the feeder link antenna 26 and DRA 24 transmit
and receive at different frequencies, a frequency translator 32 is
used to convert the frequency of transmitted/received signals,
thereby allowing signals received at the DRA 24 to be transmitted
by the feeder link antenna 26 and vice versa. In addition, the
frequency translator 32 includes a signal extraction circuit 34 for
extracting control signals that are then provided to the processor
28. The processor 28 is adapted to control the operation of the
frequency translator 32, the feeder link antenna 26 and the DRA 24
in response to the extracted control signals. The satellites 10 and
earth stations 14 will additionally include well-known and
conventional circuitry that performs call processing and
synchronization steps needed to coordinate signal traffic between
the satellites 10 and the PSTN 16.
Referring now to FIGS. 3 and 4, the DRA 24 on board the satellite
10 includes circuitry adapted to project a first and a second spot
beam 38, 40 each of which include both a beam used for transmitting
signal and a beam used for receiving signals. Although only two
spot beams 38, 40 are shown, in practice the DRA 24 actually
generates a plurality of such spot beams. The spot beams 38, 40
provide satellite voice and data service to mobile subscribers
located within a geographical region that is analogous to a cell of
a terrestrial cellular telephone system. The spot beams 38, 40,
when projected onto the earth's surface, generally have a diameter
spanning a range of several hundred kilometers and are positioned
such that each non-edge beam, i.e., beams not located on the edge
of the service coverage, slightly overlaps six other beams. In
addition, the satellites 10 are positioned such that the service
coverage provided collectively by the spot beams of each satellite
10 overlaps slightly with the service coverage provided by the
other satellites 10 to ensure contiguous service coverage over the
entire surface of the earth.
A typical call processing sequence initiated by, for example, the
mobile subscriber unit 11 located in spot beam 38, begins when the
mobile subscriber 11 dials the telephone number desired to be
reached. The dialed digits, which represent the destination address
of the call, are transmitted by the mobile unit 11 via the spot
beam 38 to the satellite 10 which, in turn, forwards the digits to
the earth station processor 20 via the earth based antenna 12. In
response to the received digits, the earth station processor 20
forwards the destination address to a switch (not shown) residing
at the PSTN 16 which responds by opening the channel corresponding
to the destination address. In addition, the earth station
processor 20 assigns a set of two transmission frequencies, a first
of which will be used for call transmission between the satellite
10 and the earth station 14 and a second of which will be used for
call transmission between the satellite 10 and the mobile
subscriber unit 11. The earth station processor 20 transmits
information regarding the set of assigned frequency channels to the
satellite 10. The satellite processor 28 stores the channel
information in memory 30 and causes the spot beam 38 to
transmit/receive the call on the assigned channel frequency. In
addition, the satellite 10 forwards the channel information to the
mobile unit 11. A transceiver (not shown) in the mobile subscriber
unit 11 then re-tunes to the assigned frequency to effect call
transmission. Call termination and other call processing functions
are performed in a similar manner with control emanating from the
earth station processor 20.
Referring still to FIG. 4, because the satellite 10 is non
geo-stationary, the spot beams 38, 40 are moving relative to the
mobile subscriber unit 11. It is assumed that the satellite 10 is
positioned and moving such that the mobile subscriber unit 11 may
eventually occupy a position within the adjacent spot beam 40,
provided that the call is maintained for the length of time
required for the satellite 10 to move the length of the spot beam
38. In addition, because the rapid movement of the satellite 10
relative to the earth greatly exceeds any possible movement of the
mobile unit 11, the mobile subscriber unit 11 is assumed to
eventually occupy a position within the adjacent spot beam 40
regardless of the speed at which the subscriber unit 11 may be
moving. To provide continuous, uninterrupted service to the
on-going call initiated by the mobile subscriber unit 11, the
on-going call is transferred from the spot beam 38 to the spot beam
40 in a so-called spot beam "handover" process that is controlled
by the earth station processor 20. Ideally, the handover should
occur when a boundary 42 that intersects the overlapping region of
the spot beams 38, 40 crosses over the subscriber unit 11. However,
the curvature of the earth and the angle at which the satellite 10
projects the spot beams 38, 40 onto the earth causes the spot beams
38, 40 to have an irregular, elliptical shape when mapped onto the
earth's surface. In addition, due to the high speed of the
satellite 10, various points on the ellipse are moving at different
speeds. Thus, the irregularity of the ellipses and the differential
speed of the ellipse boundaries make it difficult to track the
movement of the spot beams 38, 40 with respect to the subscriber
unit 11 in an earth based coordinate system.
To reduce the computational complexity required to track the
movement of spot beams of the type illustrated in this disclosure,
the present invention uses a satellite based coordinate system to
track the movement of the satellite 10 and the movement of the spot
beams 38, 40 relative to the subscriber unit 11. In particular, the
satellite based coordinate system is defined as a three dimensional
right angle coordinate system wherein the positive x axis is
defined as a vector pointing in the direction of the movement of
the satellite, the positive z axis is defined as a vector pointing
in the same direction as the satellite nadir and the positive y
axis is defined as a plane that forms a right angle with the x and
the z axes and that originates at the origin of the x-z plane. With
respect to the satellite based coordinate system, the satellite 10
is stationary and the subscriber unit 11 is moving. The spot beams
38, 40, when projected onto the x-y plane of the satellite based
coordinate system, are accurately represented as circles rather
than ellipses, thereby greatly reducing the complexity of the
computations used to track the movement of the subscriber unit 11
relative to the satellite 10.
As a brief overview, the method of the present invention is
implemented by the earth station 14 and employs an iterative binary
search technique to successively approximate a window of time
during which the subscriber unit 11 will move from the first beam
38 into the second beam 40. The second spot beam 40 is merely
intended to represent the spot beam into which the mobile
subscriber unit 11 moves and, depending upon the direction of
satellite movement, may actually be any of the spot beams that are
adjacent to the spot beam 38. After a call is initiated, an
interval during which the subscriber unit 11 will cross over the
boundary 42 between the adjacent beams 38, 40 is determined by
tracking the position of the subscriber unit 11 relative to the
spot beam boundary 42 in the satellite based coordinate system.
Thereafter, the duration of the interval is repeatedly adjusted
until the interval has been calculated to the desired degree of
accuracy. Once an interval having the desired degree of accuracy is
obtained, the midpoint of the interval is selected as the time at
which the processor 20 residing in the earth station 14 causes the
satellite processor 28 and the mobile subscriber unit 11 to effect
handover.
Referring now to a more detailed description of the present
invention, FIG. 5 shows the satellite 10 and a neighboring
identically equipped satellite 13. To ensure contiguous coverage of
the earth's surface, the satellite 10 is positioned so that the
service coverage provided collectively by the spot beams of the
satellite 10, referred to as the satellite footprint 45, overlap
with the footprint 47 generated by the neighboring satellite 13.
Due to satellite movement, an ongoing call initiated by the
subscriber unit 11 located within the footprint 45 will later
occupy a position within the footprint 47. To provide uninterrupted
phone service for the ongoing call, the call is transferred from
the satellite 10 to the satellite 13 by a handover process that is
referred to as satellite handover. Referring also to FIG. 6, to
determine when to perform satellite handover, the processor 20 of
the earth station 14 (shown in FIGS. 1 and 2), according to the
method of the present invention, determines an angle of elevation
.theta. between the subscriber unit 11 and the satellite 10. The
angle is defined by a vector 51 drawn from the satellite 10 to the
subscriber unit 11 and a vector 52 that is tangential to the
earth's surface at the location of the subscriber unit 11. The
magnitude of the angle of elevation .theta. is then compared to a
threshold angle .phi.. If the angle of elevation .theta. is less
than the threshold angle .phi. then the call is transferred to the
neighboring satellite 13. For illustrative purposes, an elevation
contour corresponding to a threshold angle of 10 degrees is shown
in FIG. 5 such that a subscriber located within the region bordered
by the elevation contour and the outer edge of the footprint 45 is
considered a satellite handover candidate.
Referring now to FIG. 7, there is illustrated a flow diagram
embodying the method of the present invention. The disclosed method
used to determine when handover should occur is executed by the
processor 20 residing in the earth station 14, whereas the actual
steps implemented to transfer the call are executed by the earth
station processor 20 operating in conjunction with the satellite 10
and the mobile subscriber unit 11. To aid in the description, spot
beam handover will be described with reference also to FIG. 4, and
satellite handover will be described with reference also to FIG. 5.
Control begins at a block 100 when a call is initiated by the
mobile subscriber unit 11 geographically located within the
originating spot beam 38 generated by the satellite 10, which, due
to the orbit and velocity of the satellite 10, will subsequently be
located in the future spot beam 40. As will be understood by one
having ordinary skill in the art, control may also begin at the
block 100 after an on-going call is transferred into the beam 38 in
preparation for the next handover to the beam 40 or when a
terrestrial system caller initiates a call to the mobile subscriber
unit 11 located in beam 38. It will be further understood that,
regardless of the origin of the call, i.e., mobile subscriber unit
11 or PSTN16, at the start of the method, the call is initially
associated with a first spot beam (e.g., spot beam 38), such that
communication between the satellite and mobile subscriber unit 11
is conducted via this first spot beam. Moreover, at a time in the
future, the subscriber unit 11 will be associated with a second
spot beam, in this example spot beam 40, in the same manner.
Next, control passes to a block 110 wherein the processor 20
estimates an interval, designated T.sub.interval, during which the
subscriber unit 11 is expected to cross over the boundary 42
between the originating beam 38 and the future beam 40. The
beginning of T.sub.interval is denoted T.sub.start and is assigned
the value of the time at which the method of the present invention
was invoked which will typically correspond to the time at which
the call was initiated or the time at which a call setup procedure
was initiated. A call setup procedure includes the conventional and
well-known steps that occur upon call initiation or call transfer
to define various parameters of the call transmission. Next, using
the velocity of the satellite 10 and the diameter of the spot beams
38, 40, the maximum amount of time that will be required for the
satellite 10 to traverse the distance corresponding to one spot
beam is determined and denoted T.sub.max. To obtain the end of the
interval, denoted T.sub.end, the time T.sub.max is added to the
time of interval start, T.sub.start. Note that the subscriber unit
11 is located in the originating beam 38 at T.sub.start and,
because the duration of T.sub.end is selected to be long enough for
the satellite 10 to have moved one beam length, it is known that at
the time, T.sub.end, the subscriber unit 11 is located in the
future beam 40. Thus, T.sub.interval represents an estimate of the
time interval during which the subscriber unit 11 will move from
the originating spot beam 38 into the future spot beam 40. Finally,
the processor 20 stores the value of T.sub.end for subsequent
processing by setting a variable T.sub.save equal to T.sub.end
(T.sub.save =T.sub.end). Note, however, that the adjacent spot beam
into which the mobile subscriber will move is not yet known, so
that at this point, the future spot beam 40 represents any one of
the spot beams that is adjacent to the spot beam 38.
Next, control passes to a block 120 where the processor 20
retrieves information from the associated memory 21 regarding the
position of the satellite 10 and the position of the mobile
subscriber unit 11 at the time of call initiation. The earth
station processor 20 then uses this retrieved information which may
include, for example, the measured signal delay between the
satellite 10 and mobile subscriber unit 11 and the Doppler effect
experienced at the mobile subscriber unit 11, to calculate the
position of the satellite 10 and the position of the mobile
subscriber unit 11. To aid in this calculation, the central
satellite control center 15 provides information regarding the
position of the satellite 10 and the movement of the satellite 10
to the earth station 14 on a daily basis. Of course, it will be
understood by one having ordinary skill in the art that the
positions of the mobile subscriber unit 11 and the satellite 10 may
be determined using any alternative means known in the art and need
not be performed exclusively by the processor 20 but may instead be
calculated by, for example, the mobile subscriber unit 11, provided
that the calculated positional information is thereafter
transmitted to the processor 20.
The processor 20 then uses the positional information retrieved
from the memory 21 to determine the position of the subscriber unit
11, denoted PosSub.sub.ECEF, and the position of the satellite,
denoted PosSat.sub.ECEF (t), which are defined relative to an earth
centered earth fixed ("ECEF") coordinate system that rotates with
the earth. In the three dimensional ECEF coordinate system the
positive x axis is defined as the vector emanating from the center
of the earth and intersecting 0.degree. longitude and 0.degree.
latitude, the z axis is defined as the line that intersects the
center of the earth and that extends through the north and south
poles and the y axis is defined as the line that intersects the
center of the earth and occupies a position such that the y axis
forms a right angle with both the x and z axes.
Because the speed of the satellite 10 dwarfs the speed of the
subscriber unit 11, the subscriber unit 11 may be assumed
stationary to simplify processing without greatly affecting
accuracy. As a result, the position of the mobile subscriber unit
11, PosSub.sub.ECEF, is determined only at the time of call setup
and is assumed fixed for the duration of the call. Of course,
should such positional data be available, it may be incorporated
into the method of the present invention by replacing the
stationary position vector PosSub.sub.ECEF with a time varying
position vector PosSub.sub.ECEF (t).
Next, at a block 130, the processor calculates a unit vector
PosSub.sub.s (t) at t=T.sub.start and at t=T.sub.end. The vector
PosSub.sub.s (t), which originates at the satellite 10 and points
toward the subscriber unit 11, is used to represent the position of
the subscriber unit 11 relative to the satellite based coordinate
system and is calculated as follows: ##EQU1##
where M.sub.R (t) is a three by three dimensional rotational
transformation matrix defined as follows: ##EQU2##
and where VelSat.sub.ECEF (t) is the velocity of the satellite.
Next, at a block 140, the processor 20 determines the angle of
elevation, .theta..sub.elev (see FIG. 6), of the subscriber unit 11
at the time T.sub.end which will be used later to determine whether
satellite handover should occur. To calculate .theta..sub.elev, the
processor 20 uses the position of the subscriber unit 11,
PosSub.sub.ECEF, and the position of the satellite 10,
PosSat.sub.ECEF (t) as follows: ##EQU3##
where t=T.sub.end.
At a block 150, to determine whether satellite handover is
necessary, the processor 20 compares .theta..sub.elev, to the
threshold angle, .theta..sub.TH. If the comparison reveals that
.theta..sub.elev is greater than .theta..sub.TH, then satellite
handover is not required and control passes to a block 160.
At the block 160, to determine the position of the subscriber unit
11 relative to the positions of the adjacent spot beams 40 and the
originating spot beam 38, the processor 20 uses standard
geometrical methods to project the vectors PosSub.sub.S(T.sub.end)
and PosSub.sub.S (T.sub.start) onto the x-y plane of the satellite
based coordinate system. Referring also to FIG. 8 which shows the
originating spot beam 38 and adjacent spot beams 40 projected onto
the x-y plane of the satellite based coordinate system in units of
degrees, the point indicated with the reference numeral 58
represents the point at which the vector PosSub.sub.S (T.sub.start)
projects to the x-y plane and the point indicated with the
reference numeral 60 represents the point at which the vector
PosSub.sub.s (T.sub.end) projects to the x-y plane. A center 62 of
the originating spot beam 38, denoted CTR.sub.origin, and a center
64, 65, 66, 67, 68 and 69 of each of the adjacent spot beams 40,
denoted CTR.sub.future, are used to calculate the distances between
the position of the subscriber at T.sub.end and each of the beam
centers 62, 64, 65, 66, 67, 68 and 69 as follows: ##EQU4##
where Dist.sub.CTRORIGIN represents the distance between the
subscriber at time T.sub.end and the center of the originating beam
38 and where Dist.sub.CTRFUTURE represents the distance between the
subscriber at time T.sub.end and the center of each of the possible
future beams 40. Of course a plurality of different values for
Dist.sub.CTRFUTURE must be calculated, each pertaining to a
different one of the centers of the spot beams 40. The plurality of
Dist.sub.CTRFUTURE values may be distinguished by adding subscripts
such as, for example 1,2,3 etc. to the various values of
Dist.sub.CTRFUTURE. For example, the distance between the
subscriber at T.sub.end and the center 64 may be represented with
Dist.sub.CTRFUTURE1 and the distance between the subscriber at
T.sub.end and the center 65 may be represented by
Dist.sub.CTRFUTURE2, etc.
Control then passes to a block 170 where the processor 20 tests to
determine whether the subscriber unit 11 has crossed the boundary
42 between the orignating beam 38 and any of the adjacent beam 40
during T.sub.interval. Because the boundaries a between the
originating spot beam 38 and the adjacent spot beams 40, which are
indicated by the reference numeral 42, are located equidistant from
the centers 62, 64, 65, 66, 67, 68 and 69 of the beams 38, 40, the
magnitude of Dist.sub.CTRorigin is compared to the magnitude of
each value of Dist.sub.CTRfuture to determine whether the
subscriber unit 11 has passed any of the boundaries 42 during the
interval. In the event that the subscriber unit 11 is closer to the
center of the originating beam 38 such that Dist.sub.CTRorigin is
less than any of the values of Dist.sub.CTRfuture, then the
subscriber unit 11 has not crossed over any of the boundaries 42 at
the time T.sub.end. Conversely, if Dist.sub.CTRorigin is greater
than any of the values of Dist.sub.CTRfuture, then the subscriber
unit 11 has crossed over one of the boundaries 42 lying between the
originating beam 38 and one of the adjacent beams 40 at the time
T.sub.end. If the value of Dist.sub.CTRorigin is greater than only
one of the values of Dist.sub.CTRfuture then the adjacent beam
corresponding to that particular value of Dist.sub.CTRfuture is
identified as the future beam 40 in which the subscriber will be
located at the time T.sub.end. If, instead, the value of
Dist.sub.CTRorigin is greater than more than one of the values of
Dist.sub.CTRfuture then the adjacent beam corresponding to the
lowest value of Dist.sub.CTRfuture is identified as the future beam
40 in which the subscriber will be located at the time T.sub.end.
For illustrative purposes only, in FIG. 8 the subscriber unit 11 is
shown occupying a position in the future beam 40 having the center
64 at the time T.sub.end as indicated by the point 60. Once the
future beam 40 into which the subscriber is moving has been
identified, then control passes to a block 180. At the block 180, a
handover flag, FLAG, is set to 1 (FLAG=1). If at the block 150, it
has instead been determined that .theta..sub.ELEV is less than
.theta..sub.TH, thereby indicating that satellite handover is
appropriate, then, after the block 150 control passes to the block
190 where the FLAG is cleared (FLAG=0). After blocks 180 and 190
control proceeds to a block 200.
Thus, at the block 200, it is known that either spot beam handover
or satellite beam handover is appropriate during T.sub.interval but
the precise time of handover within that interval is not known.
Moreover, if it has been determined that spot beam handover is
appropriate, the future beam 40 into which the mobile subscriber
unit is moving has been uniquely identified among the adjacent
beams 40. To best estimate the time of handover, the midpoint of
T.sub.interval is selected as the time at which handover will occur
and is designated as T.sub.mid. Thus, T.sub.mid is, at most,
inaccurate by T.sub.interval /2. Therefore, at a block 200, the
processor tests the accuracy of T.sub.mid by comparing the length
of T.sub.interval to a predetermined threshold accuracy T.sub.acc
to determine whether the time to handover, T.sub.mid, has been
calculated to the desired degree of accuracy.
If the length of the interval, T.sub.interval, is greater than
T.sub.acc therby indicating that the desired degree of accuracy has
not been reached, then control passes to a block 210 where the
processor reduces the duration of T.sub.interval to more accurately
pinpoint the time of handover. Prior to reducing T.sub.interval,
however, the processor 20 stores the value of T.sub.end as
T.sub.save (T.sub.save =T.sub.end). Then, to reduce T.sub.interval,
the processor 20 changes the time T.sub.end to occur earlier as
follows:
Control then returns to the block 130 and the blocks subsequent
thereto as described herein. Note that during subsequent
iterations, the distance repersented by the variable
Dist.sub.CTRfuture is only calculated with respect to the center of
the identified beam boundary.
If at the block 170, it is determined that the subscriber unit 11
has not passed over the beam boundary 42 during T.sub.interval, it
is assumed that T.sub.end was selected before boundary crossover
and control passes to a block 220. At the block 220, T.sub.start is
set equal to T.sub.end and T.sub.end is set equal to the previous
value of T.sub.end, T.sub.save. Recall that in the previos
iteration it was determined that the previous value of T.sub.end
occurred after boundary crossover and, during the current
iteration, it has been determined that the current value of
T.sub.end occurs before crossover. Thus, adjusting T.sub.start and
T.sub.end in this manner ensures that the subscriber has passed the
beam boundary during T.sub.interval.
After T.sub.interval has been adjusted at the block 220, control
again returns to the block 130 and blocks subsequent thereto as
described herein.
If at the block 200 the desired level of accuracy has been achieved
such that T.sub.interval is less than the value of T.sub.acc, then
control passes to the block 230 where the processor 20 calculates
the actual time for handover, T.sub.handover as follows:
Thereafter, control proceeds to the block 240 where the processor
20 checks the value of FLAG. If FLAG is set to one (1), thereby
indicating that spot beam handover is appropriate, then control
proceeds to a block 250 where the processor 20 effects spot beam
handover. At the block 250, the processor 20 sends a control signal
to the feeder antenna 12 residing at the earth station 14 which, in
turn, relays the signal to the satellite 10 that is transmitting
the on-going call. The satellite feeder antenna 12 receives the
transmitted signal which is subsequently demodulated at the
frequency translator 32. The signal extractor 34 supplies the
demodulated control signals to the processor 28 which, in response
to the control signals, causes the DRA 24 to transmit the control
signal to the subscriber via spot beam 38. In addition, information
regarding the time of handover and the new frequency at which the
call will be transmitted by the future spot beam 40 is transmitted
by the satellite DRA 24 to the mobile subscriber unit 11.
If, instead, FLAG is set to zero (0), then control proceeds to a
block 260 where the processor 20 effects satellite handover.
Satellite handover is initiated when the processor 20 sends a
control signal to the feeder antenna 12 which transmits the signal
to the feeder antenna 26 residing at the originating satellite 10.
The signal received at the satellite feeder antenna 26 is then
demodulated at the frequency translator 32 and thereafter extracted
by the signal extractor 34. The signal extractor 34 supplies the
demodulated control signal to the processor 28 which responds to
the signal by transmitting it to the subscriber unit 11. The DRA 24
of satellite 10 transmits the information regarding the time of
handover and the new frequency at which the future satellite 13
will transmit the call to the mobile subscriber unit 11. After the
call has been transferred the program begins again at the block 100
to prepare for the next call transfer.
Because the actual operations of spot beam handover and satellite
handover may be performed in any of a variety of well known ways,
the steps performed at the blocks 250 and 260 are provided for
illustrative purposes only. Typical handover procedures may include
any number of steps such as, for example, ending transmission at
the originating beam 38 and beginning transmission at the future
beam 40 or determining the new unused frequency at which the call
will be carried after handover or it may instead involve other
processing steps to ensure signal quality during call transfer.
It will also be understood by one having ordinary skill in the art
that the method of the present invention provides geometric
simplicity and processing speed gained by tracking the movement of
the subscriber in the satellite based coordinate system and using
the movement to estimate the time of handover. In addition,
although calculated with reference to the earth based coordinate
system herein, the angle of elevation may instead by determined
relative to the satellite based coordinate system.
While the method of the present invention has been described with
reference to a specific set of steps, which are intended to be
illustrative only, and not to be limiting of the invention, it will
be apparent to those of ordinary skill in the art that changes,
additions, and/or deletions may be made to the disclosed embodiment
without departing from the spirit and scope of the invention.
* * * * *